Every time I start playback, a very loud and low 5 seconds long tone is sounding, unrelated to the score itself. That pitch isn't notated anywhere but it is always the same pitch sounding when starting the playback.

I am using the Play engine with the EWQLSO Gold solo strings for which I created Expression Maps. This tone doesn't happen with the Default Dorico playback library so it's probably Play related. Any idea how to fix this?

On my system, the EW Play installer deposits two Play 5.0.4 VST's: one named "play_VST_64" which is VST 2.x and one named "Play" which is VST3. I'd look for both, determine which one Dorico is loading, and then try the other one. Also, double check Dorico's menu and see if there are two items that start with "Play" in the list of VSTi plugins.